The goal of the proposed application is to investigate the effects of Orthostatic Hypotension (OH) on cognition in Parkinson's Disease (PD). Prevalence estimations of OH in PD range from 20 to 60%, indicating a need for a full understanding as to its effects. The most likely etiology is a cardiac sympathetic neuropathy affecting heart-rate variability and cardiac output. Dopaminergic medications may also play a role in exacerbating OH in PD. To date, no study has investigated the acute effects of OH on cognition in PD. This study will use objective measures of autonomic function, including electrocardiogram, arterial pressures, and transcranial Doppler flowmetry to assess the effects of a sixty-degree head-up tilt on sympathetic activation and gravitational compensatory responses. These physiological measurements will be analyzed against several neuropsychological test measures administered in both the supine and tilted positions. The participant groups will include PD patients with OH, PD patients without OH, and demographically matched normotensive control participants. Previous research suggests that OH will lead to cerebral hypoperfusion in the upright position, inducing symptoms of lightheadedness, dizziness, and fatigue. It is hypothesized that PD participants with OH will show within- subject detriments in several domains of cognitive function while tilted, and that PD participants with OH will demonstrate poorer overall performance than both PD participants without OH and normal control participants while supine. Test measures will include assessments of working memory, attention, executive function, as well as assessments of mood and activities of daily living. A variety of cognitive deficits have been reported in PD, and a small body of research suggests that PD patients with OH demonstrate poorer performance across several domains. By understanding the effects of OH on cognition in PD, emphasis on controlling compensatory responses to postural change will allow for an increased quality of life with more independence. Furthermore, maintaining adequate cerebral blood flow may allow for better cognitive abilities for the duration of the disorder.